1. Field of the Invention
This invention relates generally to methods, systems and apparatus for managing digital communications systems. More specifically, this invention relates to estimating the configuration of a group of channels or lines in a communication system such as a DSL system.
2. Description of Related Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines (referred to as loops and/or the copper plant). Telephone subscriber lines can provide this bandwidth despite their original design for only voice-band analog communication. In particular, asymmetric DSL (ADSL) can adjust to the characteristics of the subscriber line by using a discrete multitone (DMT) line code that assigns a number of bits to each tone (or sub-carrier), which can be adjusted to channel conditions as determined during training and initialization of the modems (typically transceivers that function as both transmitters and receivers) at each end of the subscriber line.
“xDSL” and “DSL” are terms used to generally refer to digital subscriber line equipment and services, including packet-based architectures, such as ADSL, HDSL, SDSL, SHDSL, IDSL VDSL and RADSL. DSL technologies can provide extremely high bandwidth over embedded twisted pair, copper cable plant. DSL technologies offer great potential for bandwidth-intensive applications.
ADSL or asymmetric digital subscriber line services generally use existing unshielded twisted pair copper wires from a telephone company's central office (CO) to a subscriber's premise. ADSL modems at both the CO and remote locations send high-speed digital signals over the copper wires and may be capable of providing a downstream bandwidth of about 1.5 Mbps-6.144 Mbps (8 Mbps in ADSL1 and used in Japan and China already), and an upstream bandwidth of about 32 Kbps-640 Kbps with loop distances ranging to 5.5 km.
HDSL or high bit rate DSL provides a symmetric, high-performance connection over a shorter loop, typically requires two or three copper twisted pairs, and is capable of providing both upstream and downstream bandwidth of about 1.5 Mbps over loop distances of up to about 3.7 km. SDSL or single line DSL provides a symmetric connection that matches HDSL data rates using a single twisted pair, but operates over a shorter loop of up to about 3.0 km. VDSL or very high bit rate DSL typically is implemented in asymmetric form, as a very high speed variation of ADSL over a very short loop. Specifically, target downstream performance is typically about 52 Mbps over local loops of 300 m, 26 Mbps at 1,000 m, and 13 Mbps at 1,500 m. Upstream data rates in asymmetric implementations tend to range from about 1.6 Mbps to about 2.3 Mbps. VDSL also offers symmetric data rates of typically 10-25 Mbps. Newer versions of VDSL known as VDSL2 promise symmetric data rates of 100 Mbps and downstream rates to 150 Mbps in asymmetric configurations. Additionally, there are a small number of nonstandard RADSLs or rate adaptive asymmetric DSLS, which, like ADSL, provide a dynamic data rate that adapts to the length and quality of the line (and used a line transmission method that is now nearly defunct in DSL called QAM or CAP). These versions of DSL utilize a packet-based approach that does away with the line-grabbing practice of circuit switched networks. This packet-based approach works well in a variety of situations.
DSL services are much more dependent on line conditions (for example, the length, quality and environment of the copper loop) than traditional telephone services, which typically use a bandwidth including frequencies up to about 4 kilohertz compared to DSL services which utilize a bandwidth including frequencies sometimes over 1 MHz. While some local loops are in great condition for implementing DSL (for example, having short to moderate lengths with minimal bridged taps and splices) many local loops are not as suitable. For example, local loop length varies widely. Moreover, the wire gauge for a local loop may not be consistent over the length of the loop, having two or more different gauges spliced together. Still further, many existing local loops have one or more bridged taps (a length of wire pair that is connected to a loop at one end and is unconnected or poorly terminated at the other end), and some local loops have bad splices (for example, a splice that is loosely connected). This type of line information (for example, wire gauge information, bridged-tap information, segment information, bad splice information and load coil information) is important to the evaluation of DSL systems and configurations. Another important class of line conditions is the noise measured on the line, which can be caused by radiation from other DSLs (“crosstalk”), radio ingress of AM or amateur radio stations, thermal noises in the line or receiver analog components, various appliances at the home, electronic equipment in the loop plant or at the central office. These types of noises can vary from time to time and be relatively stationary, impulsive or a combination of both. This type of information also can be important for the evaluation of DSL systems and configurations.
The different conditions and configurations of these loops, including how they are arranged and operated within bundles or binders from the telephone company CO and other locations, mean that every group of DSL loops is different and thus behaves differently. Information may exist about individual lines, or can be determined using earlier techniques (for example, evaluation using voice-band measurement and loop-qualification methods). However, this information fails to take into account the interaction among lines (active and inactive), including interactions such as crosstalk (that is, unwanted interference and/or signal noise passed between adjacent lines that occurs because of coupling between wire pairs when wire pairs in the same or a nearby bundle are used for separate signal transmission). Moreover, the accuracy of some of this information is questionable; it has been found that line quality varies widely, even among lines in the same group. Further, voice-band measurements do not always accurately reflect the DSL environment of loops. Therefore, techniques that evaluate a single line in each binder or other group, for example, and then extrapolate that information to all other lines in such a group, may not provide accurate information about those other lines or even the evaluated line itself.
Other techniques include characterizing DSL transmission lines using time-domain reflectometry, in which a predetermined test signal is sent from a point of origin to a DSL transmission line, the line reflects a portion of the signal back to the point of origin, and the reflected signal is analyzed to determine transmission-line characteristics. In other situations, a reference loop might be analyzed and/or characterized to generate a transfer function and to model the effects of attenuation, noise, etc. on signals in the reference loop. Typically, one reference loop is selected in each binder or other group of lines and evaluated.
However, these techniques for evaluating a single loop or line do fail to take into account the environmental operation of these lines. That is, there are environmental conditions that affect line performance beyond the behavior of the line alone. Testing a single reference loop may develop some basic information about the line itself, but such information does not assist in the understanding and implementation of optimized services to many users who are using the grouped lines contemporaneously.
Another problem with the testing, monitoring, and maintenance required for successful DSL deployment is the fact that different parties frequently use and operate adjoining DSL lines. For example, some lines in a CO might be operated by an ILEC (Incumbent Local Exchange Carrier), which utilizes its own operational and usage rules and systems. Other lines in the same binders and/or other groupings might be operated by one or more CLECs (Competitive Local Exchange Carrier), which are in direct competition with the ILECs in the marketplace, and which likewise have their own operational and usage rules and systems. The exclusionary and competitive nature of these situations, and others like them, mean that there is little opportunity to obtain specific information about the DSL line environment.
Systems, methods and techniques that permit modeling of DSL systems, including DSL binders and other groups, would represent a significant advancement in the art. In particular, management systems may provide only limited information nominally on the line and a system that could infer substantially more information from that limited information would represent a considerable advancement in the field of DSL service rates and associated ranges.